Radial Lip Seals for Propulsion Shafting

An outboard seal arrangement on an oil-lubricated stern tube serves two primary purposes: to prevent lubricating oil from leaking into the sea and to stop seawater from entering the stern tube. This arrangement typically consists of several elastomeric lip seals fitted in sequence. These seals are mounted within a casing attached to the aft end of the stern tube and press against a smooth, hardened metal liner fitted on the propeller shaft. A garter spring wrapped around the lip of each seal provides continuous radial force to maintain the seal's contact with the shaft liner. The seals are usually made from durable materials like nitrile rubber or fluoroelastomer (Viton), chosen for their resistance to oil and temperature variations.

In a typical arrangement, multiple seal rings are used. The outermost seal acts as a seawater barrier, while the inner seals retain the lubricating oil. The outboard seals dissipate heat to the surrounding seawater, while the inner seals transfer heat to the lubricating oil through convection. The entire seal assembly includes key components such as a flange, cover rings, intermediate rings, and the shaft liner, all designed to work together to create a reliable barrier.

Loss of oil from the stern tube header tank is a serious condition as it leads to insufficient lubrication and increases the risk of seawater ingress into the stern tube bearings. The following actions should be taken:

1. Immediate checks:
   • Verify oil level in the header tank.
   • Top up with the correct grade of lubricating oil if required.
2. Inspection for leakage:
   • Check stern tube seal chambers and drain tanks for signs of oil leakage or seawater ingress.
   • Identify the source of failure (seal wear, damage, or liner scoring).
3. Monitoring:
   • Observe oil level alarms, leakage indication systems, and bearing temperature alarms.
4. Operating adjustments:
   • If leakage is severe, reduce shaft speed to minimize further oil loss.
   • Stop the main engine if necessary to prevent bearing damage.
5. Leakage control measures:
   • Temporarily lower the oil level in the header tank to reduce leakage pressure.
   • Use more viscous oil (if permitted) to slow down the leakage rate.
6. Repair arrangements:
   • Plan for emergency seal repairs, either through underwater maintenance by divers or during dry docking.
   • Ensure spare sealing elements and liners are available.
7. Continuous lubrication assurance:
   • Maintain positive oil pressure at the seals via gravity feed or auxiliary pumps.
   • Drain any accumulated water/oil mixtures in seal chambers regularly.

ALTERNATE ANSWER:

The loss of oil from the stern tube header tank is a critical issue that can lead to insufficient lubrication for the stern tube bearings and potential seawater ingress. This can cause severe damage to the bearings and the propeller shaft. Immediate and reasoned actions are essential to mitigate the risk.

Here are the possible actions and the reasons behind them:

• Immediately check and top up the oil level: The most direct action is to restore the oil level in the header tank with the correct grade of lubricating oil. This re-establishes the hydrostatic pressure, which is essential to prevent seawater from entering the stern tube.
• Reduce shaft speed or stop the engine: If oil loss is significant and ongoing, a reduction in propeller shaft speed or a complete stop is necessary. This lessens the pressure and heat on the seals, reducing the leak rate and minimizing the risk of bearing damage due to poor lubrication.
• Inspect seal chambers and drain tanks: Regularly checking the drain tanks and seal chambers for an abnormal mixture of oil and water helps diagnose the location and severity of the leak. For example, a large amount of milky-white fluid indicates significant seawater ingress, while an excessive amount of clear oil points to an oil leak.
• Consider temporary pressure adjustments: As a temporary measure, the header tank's height may be adjusted to change the hydrostatic pressure. In some cases, a more viscous oil might be used to reduce the leakage, but this is a short-term solution and should only be done if the oil is compatible with the system.
• Arrange for repair or replacement: The underlying issue—a failed seal—must be addressed. This requires planning for either an underwater seal replacement by divers while the vessel is afloat or, for a more permanent and thorough repair, drydocking the vessel. This ensures the long-term integrity of the sealing system.
• Maintain positive pressure: Ensuring a continuous and positive oil pressure within the stern tube system is paramount. This pressure, supplied by the header tank or a pump, creates a positive differential pressure that actively prevents seawater from breaching the seals. This is the fundamental principle of preventing water ingress.